I've put together a quick CLI for macOS and a very naïve benchmarking suite to see what the gains are like. Compared to optimised PNG, it's worse most of the time but the interesting thing is when it is better. I'm not sure there's a hard and fast rule, at least not that I've found so far.

I really needs more benchmarks, especially decompression time. Also the sizes are interesting for very small images, but for real images, there are maybe better lossy variants:

  nz_scene - PEP = 73.542 bytes,
       lossy-PNG = 43.557 bytes,
      lossy-WEBP = 26.654 bytes,
  lossy-mozcjpeg = 15.716 bytes

So it's not about filesize here, it must be decompression speed.